Boost has existed since 1999-2000, which aligns with ES3.
As for solid and well-understood... Douglas Crockford wrote a book on how "solid" Javascript is, called "Javascript: The Good Parts". One of his most common refrains about JS was "Javascript is The World's Most Misunderstood Programming Language". [0][1]
ES5 came out in 2009. ES6 was standardized after React came out.
I'd complain the same way if it were a C++ project requiring Boost or something.